The present invention relates to fluidized bed combustors in general and, more particularly, to a fluidized bed combustion apparatus for retrofit in a residential heating system, or in light industrial heating applications.
The use of coal for residential heating or for light industrial heating applications is increasing as the availability of other fuel sources, principally oil and natural gas, decreases and the price of each increases. Woodburning, while enjoying a rapid increase in popularity, is expected to be restricted by the availability of wood. Wood fuel costs, problems of safety and emerging problems of air pollution combine to limit further expansion of wood as a viable source of fuel for domestic heating systems.
On the other hand, proven coal reserves have been conservatively estimated as sufficient to ensure world energy supply for at least 200 years. Unfortunately, with the conversion to oil since the mid-century, coal combustion development was halted and has only recently surfaced as a focus for major research and development. These research efforts were oriented to solutions to those problems which originally led to the decline in the use of coal and to its replacement by oil. In particular, the problems requiring solution were oriented to quality, storage, handling and transportation, solid waste disposal and, more recently, air pollution. Overall, coal could not compete with the convenience of oil. To allow return to popular use, coal combustion units must therefore be efficient and safe, of low capital cost, flexible in operation and performance, able to accept various types and quality of coal feed, including raw coal and coal/water, coal/oil slurries, and be clean in operation.
Fluidized bed combustion has been identified for over the past two decades as one of the promising technical developments to address the problem of efficiency, cleanliness, cost effectiveness and controllability.
Fluidized bed combustion has been applied on an industrial scale for more than a decade. There are about 200 units in operation in the western world and it is reported that there are some 2,000 units in China. More units produce process steam and hot water, but some are used in other applications such as grass drying. In recent years, there has been increasing interest in fluidized bed combustion for utility application. This is primarily because fluidized bed combustion offers a "front-end" alternative to flue gas scrubbing for reduction of SO.sub.x emissions. The capability arises because a coal fired fluidized bed combustor can be operated at temperatures in the range of 750.degree. C. to 950.degree. C.
Another advantage that the fluidized bed combustor has that makes it attractive in a utility application is that the heat transfer coefficients for surface in the bed can be several times greater than are obtained in conventional boilers. Therefore a fluidized bed combustion boiler can be smaller than a conventional boiler of the same capacity.
Further, the fluidized bed combustor can utilize low and variable quality fuel. For example, it can burn coal that contains up to seventy percent ash. The ability to burn low quality fuel makes fluidized bed combustion viable at the domestic scale.
Prior art fluidized bed furnaces, such as those used to produce steam and hot water, were not suited for application to residential or small industrial scale applications. These prior art fluidized bed furnaces were generally too sluggish for either space heating or small industrial process heating which requires the rapid response to heat load demand and rapid cycling capacity.
Most recently, fluidized bed furnaces have been developed for use in a residential heating system. U.S. Pat. No. 4,416,418, issued on Nov. 22, 1983 to Goodstine et al. discloses a fluidized bed combustion system suitable for use in a residential heating system, comprising a fluidized bed combustor housing an insulated adiabatic combustion chamber wherein crushed fuel is burned to generate hot flue gas, a heat exchanger disposed outside of the combustor in the flow path of the flue gas, a particulate collector for removing fine particles entrained in the flue gas before the flue gas is vented to the atmosphere, and an induced draft fan for venting the flue gas to the atmosphere, drawing fluidized air and recirculating flue gas through the combustion chamber. Electric heating means are disposed within the fluidized region of the combustion chamber to heat the slumped bed during shutdown to maintain bed temperature above the ignition point.
The fluidized bed furnace described by Goodstine et al employs electrical resistance heaters immersed in a horizontal disposition within the particulate material of the fluidized bed. We have discovered that the electrical resistance heaters within the combustion bed of a device as disclosed in Goodstine are subject to a rapid erosion of the heater elements which may result in the failure of the entire heating system.
Additionally, the Goodstine et al furnace utilizes an induced draft fan. Thus the gas duct which recirculates part of the combustion stream to the inlet air plenum is at a positive pressure with respect to the interior of the combustor. This arrangement results in that the flue gas is at a positive pressure with respect to the outer atmosphere. Accordingly, a leak in the flue gas system would permit combustion gases to enter the household.